It is well known that electric power delivered during periods of peak demand can cost substantially more than off-peak power.
Various schemes are known, often referred to as “smart” appliance or “smart meters” to enable a given consumer to benefit from different prices for power. US patent application 2007/0276547, for example, seeks to optimize control of energy supply and demand by scheduling the control of energy consumption devices on the basis of variables relating to forecast energy supply and demand and by providing local battery storage and alternative energy sources (e.g., photovoltaic cells) which sell energy to the power grid during periods that are determined to correspond to favourable cost conditions. The instantaneous energy usage of a premises or a business are monitored and batteries are used to reduce demand from the grid. Solar power production is predicted in 15-minute intervals based on the previous 15 minutes. Non-critical energy uses (“deferrable loads”) such as a commercial freezer can be temporarily shut off (it is described that freezers can be shut off for two hours at a time whereas refrigerators can be shut off for only 15 minutes at a time and should operate for three hours out of every four). The 15-minute estimates can be replaced by a forecast pattern of use that is selected based on predicted weather, scaled depending on actual use (demand).
Whereas such schemes contribute to reducing demand from the grid at peak time, nevertheless, from the point of view of the grid operator, peaks will be experienced and it is nevertheless expensive to have to provide for sufficient capacity to meet these peaks.
One solution to this problem is a pumped-water energy storage facility such as that installed in Dinorwig in Wales. At this facility, excess energy provided by power generators in periods of low electricity demand is used to pump water from a lower reservoir to a higher reservoir. In periods of high electricity demand, the pumped water is allowed to flow from the upper reservoir via a conventional hydroelectric generating turbine to the lower reservoir to generate additional power to assist to match sudden additional electricity demand to the electricity power network. Whereas the response of such a facility can be quite fast, it is not necessarily fast enough to maintain voltage and frequency specifications at a point quite remote from the facility. It is also an expensive solution in terms of infrastructure and environmental impact and it is relatively energy inefficient.
Clean forms of energy generation, such as wind and solar, suffer from intermittency, which can be quite rapid, with changing wind gusts or cloud occlusion. These and other factors can contribute to grid instability, which wastes energy, both directly and indirectly, for example by requiring power generators and or encouraging power consumers to install expensive or inefficient forms of backup generation.
US patent application US2009/0200988 and U.S. Pat. No. 5,642,270 proposes an improvement by aggregating electric vehicle batteries to meet medium- and large-scale needs of power services, and an arrangement is described in which a vehicle battery and associated power electronics within the vehicle can provide local power backup power during times of peak load or power outages.
Other efforts, such as GB2472280A, focus on “responsive loads” such as domestic refrigerators, air conditioning, washing machines and the like that can report to a national grid centre control room their actual availability at any given time to respond to a bidding market in which, individually, they temporarily elect periods of time in which to consume power.
When power supplied and power consumed are not equal, the supply system either accelerates (e.g., when there is a rapid fall-off in load), causing the generators to spin faster and hence to increase the line frequency, or decelerates (e.g., when there is a rapid increase in demand), causing the line frequency to decrease.
Variations in line frequency can occur due to rapid changes in supply (e.g., photovoltaic sources) as well as in demand. For this reason or for other reasons (e.g., localized pinch points in the distribution grid), it may not be possible to utilize local electricity production capacity. Hardware constraints sometimes have to be placed on local ability to feed into the grid (see US2007/0276547).
To respond to fluctuations in line frequency “regulating reserve” must be available almost immediately when needed (e.g., in as little as a few seconds to less than about five (5) minutes). Governors can be incorporated into a utility's generation system to respond to minute-by-minute changes in load by increasing or decreasing the output of individual macro generators and, thereby, engaging or disengaging, as applicable, the utility's regulating reserve. This is described in US2014/0018969, which also describes how providing electric power to the grid from storage devices such as fuel cells battery devices and energy potential systems (including stored water systems) raises new challenges. That document proposes a reporting infrastructure for control by a grid operator and proposes “active supply clients” for management of electric power available to the electric power grid, whether by generation source supply elements or by storage source supply elements (such as battery, fuel cell, compressed air, stored water or the like), with the aim of generation balancing so that storage devices serve to stabilize and regulate renewable energy resources, or with the aim of optimization according to various factors such as cost, timing, price, market conditions and the like.
These new challenges are not served by centralized grid management, not least because of latency in control systems (especially latency in client-server reporting and control arrangements). A further issue is that changes can occur at short notice in the very infrastructure to be controlled. For example, a responsive load such as a refrigeration plant may withdraw its availability as a responsive load in a predictable or unpredictable manner, thus upsetting the sought-after balance in generation. Similar considerations apply to sources and to stores.
Neither does centralized management and control address the problem of pinch-points, by which a macro-level attempt to balance supply and demand can be thwarted if there is some local node which is a limiting factor at a particular time of day. Indeed, to achieve balance at a macro-level may create new peaks in current at new points in the network and at unforeseen times. This in turn can lead to additional cost through having to increase design specification, or opportunities for cost saving can be missed in day-to-day operation, e.g., where cost of emergency maintenance exceeds normal operating costs, but cannot be avoided because of inflexibility in the control arrangements.